A bonded piston sealing (BPS: including a piston sealing, a canceller sealing, a drum sealing, and the like) of a rubber lip crosslinking-bonded type, used in clutch coupling in an automatic transmission (AT) of a vehicle such as an automobile, generally includes a metal ring that is oil-hydraulically actuated in the automatic transmission (is caused to conduct a reciprocal motion), and a rubber sealing that is crosslinking-bonded to the metal ring and that achieves a sealing action.
As described also in Patent Document 1, a sulfur crosslinking-based acrylic rubber (ACM) is applied to the rubber for the BPS.
The crosslinking density of the ACM is increased by causing secondary crosslinking therein as a feature of the polymer for the ACM to be used.